Migrate Column tests to JUnit 5 so they actually run

[jnioche]

What

ColumnTest, CURColumnTest and AzureColumnTest were written against JUnit 4 (org.junit.Test / org.junit.Assert.*), but pom.xml only declares junit-jupiter-api/engine/params — there is no junit-vintage-engine. Surefire was silently skipping all three: mvn test -Dtest=ColumnTest reported Tests run: 0, and they never showed up in the full run either.

This migrates them to JUnit 5:

• org.junit.Test → org.junit.jupiter.api.Test
• org.junit.Assert.* → org.junit.jupiter.api.Assertions.*
• dropped the now-redundant public modifiers on the classes and test methods

It also deletes CURColumnTest, which was a strict subset of ColumnTest — identical three methods, both constructing CURColumn, with ColumnTest adding testResolveIndexOptionalCachesMisses on top.

Result

mvn -o test: 356 → 363 tests, all green.

• ColumnTest 4, AzureColumnTest 3 (the +3 from the deleted CURColumnTest were pure duplicates)
• Nothing was actually broken. All ten recovered tests passed on the first run, so the production code had not drifted away from them while they were dormant.

Notes for reviewers

No assertion bodies needed changing, which is the part most likely to bite in a JUnit 4 → 5 migration:

• None of the three files used the assertEquals(String message, ...) form, whose message argument moves to last in Jupiter. Nothing to reorder.
• The two calls that looked at risk bind correctly anyway: assertEquals(3.14159, col.getDouble(r1), 1e-9) → assertEquals(double, double, double delta), and assertEquals(-1, col.resolveIndex(missing, true)) → assertEquals(int, int), since Column.resolveIndex returns int. Neither silently picks up a message overload.

The diff is otherwise purely mechanical — four import/modifier changes per file.